The invention relates to thermally-removable epoxies. More particularly, the invention relates to thermally-removable epoxies prepared using the Diels-Alder cycloaddition reaction and their method of making.
Epoxies are commercially used as solids for a wide variety of applications, including extensive use in the microelectronics industry. For example, printed circuit boards generally have epoxy in them. Epoxies are useful in that they tend to provide structural integrity, including solvent resistance and mechanical strength.
The reaction between oxirane (or epoxide) groups and amines form the basis for many technically important coatings, adhesives, and structural composites, especially those containing reinforcing fibers, known as epoxies. These epoxy materials are generally formed from two liquid components, one containing oxirane (epoxide) end-groups and the other primary or secondary amines. These components are mixed just prior to use and reaction ensues at room temperature, proceeding until it becomes diffusion limited. Chemical cross-linking occurs during the reaction to form an insoluble cross-linked epoxy thermoset. A thermoset compound is a polymer that softens when initially heated, then hardens and condenses in bulk to retain a permanent shape; the thermoset material cannot be softened or reprocessed by re-heating.
The reaction of condensation is, for secondary amines: 
and for primary amines (each primary amine acts as a difunctional molecule): 
Thus dioxirane compounds (diepoxides) can be crosslinked by diamines (for example with 1,6-diaminohexane) which are tetrafunctional. The use of polyamines and polyimines will lead to gelation at very low conversions.
For composite structures, aromatic diamines are preferred, and the mixed resin after gelation is progressively heated to temperatures above the ever-increasing glass transition to induce the greatest possible extent of reaction of the amine groups with the oxirane groups, so as to avoid subsequent sensitivity to moisture.
In the formation of epoxies, the presence of hydroxy groups and amine groups promotes substantial hydrogen bonding that can occur between polymer chains. The chemical crosslinks and the hydrogen bonding inherent in epoxies cause them to be insoluble, unprocessable thermosets. Because epoxies are used to encapsulate a wide variety components that are becoming ever more costly, it would be useful to develop epoxies that are removable/reversible. This would allow for epoxy removal and component recovery; for example, for repair or component replacement.
Dickie et al. (U.S. Pat. No. 4,687,812, issued on Aug. 18, 1987) describe the formation of epoxy resins using diene functional aminoepoxy resins and dieneophile functional aminoepoxy resins. Meurs (U.S. Pat. No. 5,641,856, issued on Jun. 24, 1997) describes a remoldable cross-linked resin obtained from reacting a dienophile and a 2,5-dialkylsubstituted furan. The furans are substituted at both the 2 and 5 positions to prevent unwanted side reactions that cause irreversible crosslinking; the furans may also be substituted at the 3 and 4 positions with alkyl or alkyloxy groups. Meurs utilizes polymeric furans as the diene compounds; as polymeric solids, the reaction temperatures must be sufficiently high to allow mixing of the reactants. As shown in the examples of Meurs, the reaction temperature is 150xc2x0 C. or higher. The remolding is carried out at a temperature above 80xc2x0 C., more preferably above 110xc2x0 C., and in particular above 140xc2x0 C. The higher reaction temperature required by using solids as reactant is a significant disadvantage. lyer and Wong (U.S. Pat. No. 5,760,337, issued on Jun. 2, 1998, and U.S. Pat. No. 5,726,391, issued on Mar. 10, 1998) describe thermally reworkable binders for semiconductor devices wherein the reworkable binders comprise a crosslinked resin produced by reacting at least one dienophile with a functionality greater than one with at least one 2,5-dialkyl substituted furan-containing polymer with a. filler material. lyer and Wong (""391) also discuss that the furans are substituted at both the 2 and 5 positions to prevent unwanted side reactions that cause irreversible crosslinking. As in Meurs, polymeric furans are utilized as the diene compounds; as polymeric solids, the reaction temperatures must be sufficiently high to allow mixing of the reactants. Due to the viscosity of the polymeric furans, the reworking temperature of this system is 100xc2x0 C. and preferably from about 130xc2x0 C. to about 250xc2x0 C.
A thermally reversible curing system with lower curing and reworking temperatures would be useful in certain applications, such as in microelectronics applications where encapsulation at high temperatures might degrade the components. Useful also would be reactants that are liquid at or near room temperature to provide easier encapsulation processing. This property would also facilitate de-encapsulation.
According to the method of the present invention, a thermally-removable epoxy can be prepared by mixing a bis(maleimide) compound to a monomeric furan compound, with the monomeric furan compound containing an oxirane group, to form a di-epoxy mixture. A curing agent, such as an amine curing agent, is added to this di-epoxy mixture and the resulting mixture is cured at temperatures from approximately room temperature to less than approximately 90xc2x0 C. to form a thermally-removable epoxy. The epoxy has the property that subsequent heating to a temperature greater than approximately 90xc2x0 C. will depolymerize the solid. The depolymerization rate is increased by heating in a polar solvent, such as n-butanol, N,N-dimethylacetamide, methyl sulfoxide, and N,N-dimethylformamide.
In one embodiment, the monomeric furan and bis(maleimide) mixture is a liquid at a temperature of less than approximately 60xc2x0 C. This facilitates rapid reaction to form the di-epoxy mixture.
In another embodiment, a second di-epoxy compound is added to the first before curing. A filler can be added to the di-epoxy compound mixture to be subsequently cured to form a syntactic foam. Again, this foam can be easily removed at temperatures less than approximately 90xc2x0 C. In another embodiment, a bubble nucleating agent, a foaming agent, and a surfactant is added to the di-epoxy mixture to form a foam precursor, where, upon heating, forms a thermally-removably epoxy foam.